Nebulizers or fog generators are commonly used for humidification of ambient air and for inhalation therapy. Treatment of certain respiratory conditions, such as asthma, by this method is the preferred treatment. Generation of stable, micron-size aerosol fogs is commonly accomplished by either air driven nebulizers or by ultrasonic generators. Electronic equipment used for ultrasonic mist generation is relatively complex, cumbersome, and is hardly portable. Conventional air driven nebulizers are, small, rugged and low in cost and may be disposable. However, the ancillary air compressor is usually large and requires AC power to function. The diaphragm pumps could be converted to DC operation, but their relative in efficiency and the attendant high power drain would dictate a rather massive battery for reasonable running times.
Portable air compressors suitable for use with commercially available inhalation therapy nebulizers generally develop about 0.4 scfm at 12 to 13 psi. This pressure range is best served by efficient positive displacement, piston or rotary vane compressors driven by efficient permanent magnetic DC motors. Available nebulizers provide optimal performance in inhalation therapy over a relatively narrow range of pressure. In air compressor applications, changes in motor excitation voltage will naturally result in variations in the pressure-volume characteristics of the pump. Experiments have shown that relatively small, i.e., 10%, changes in voltage can adversely effect the efficiency of treatment when nebulizers are coupled to air compressors driven by DC motors. In situations where complete portability is desired, secondary or storage batteries must be used as a power source for such DC motor-compressor. Practical battery systems for such an application include nickel cadmium or sealed (gelled electrolyte) lead-acid cells. The requisite air pressures and volumes can be developed by small and efficient motor-pump combinations, but at best these consume over 36 watts of electrical power. Given a minimal treatment schedule of at least four 15 minute sessions for a portable nebulizer, battery capacity, size, weight and cost become critical factors.
For cost effective applications, sealed lead-acid batteries offer a good value, albeit with penalties in weight and package volume. However, the normal voltage decay characteristic of any storage battery system presents problems if a relatively long period of constant voltage is desired, particularly at high current, without having to resort to large, heavy batteries. Constant voltage is available only over a relatively small portion of the capacity curve.